This invention relates generally to superconducting electronic devices and, more particularly, to superconducting Josephson junctions and methods for their fabrication.
A superconducting Josephson junction is a bi-stable switching device having a very thin insulating or barrier layer sandwiched between a superconducting base electrode and a superconducting counter electrode. When current supplied to the Josephson junction is increased above the junction's critical current, the device is switched from a superconducting zero-voltage state to a resistive voltage state. The resistive voltage state is switched off by reducing the current supplied to the junction to about zero. Because this switching operation can occur in as little as a few picoseconds, the Josephson junction is a very high speed switching device which is particularly useful in superconducting electronic devices.
Josephson junctions are frequently fabricated by depositing a superconducting layer, such as niobium (Nb), on an insulating substrate. The superconducting layer is patterned and etched using conventional optical lithographic techniques to form the base electrode. The insulating or barrier layer is then formed by depositing an insulating material on the base electrode or by oxidizing the surface of the base electrode. The counter electrode is then formed on the insulating layer by depositing another superconducting layer.
Although this conventional fabrication process is widely used, it has several disadvantages. One disadvantage is that conventional optical lithographic techniques, such as photolithography, are generally limited to linewidths of about a micron. X-ray or electron beam lithographic techniques have submicron linewidths, but these techniques are extremely expensive and are not presently suitable for mass production of electronic devices. Submicron linewidths are needed for fabricating Josephson junctions with small junction areas, which are desirable because they increase the speed of the device and allow more Josephson junctions to be packed onto a substrate, thus saving space and decreasing signal propagation time between the junctions.
Another disadvantage of this conventional fabrication process is that the junction region is exposed to undesirable contaminants. For example, the insulating barrier layer is formed on a base electrode which has been exposed to the atmosphere, covered with photoresist materials and subjected to the chemicals used in the etching process. Accordingly, there has been a need for a Josephson junction and a method for its fabrication which does not suffer from these disadvantages. The present invention clearly fulfills this need.